الفهرس | Only 14 pages are availabe for public view |
Abstract Keratoconus is a degenerative, non-inflammatory disorder of the cornea, characterized by central and para-central thinning and subsequent ectasia. This distortion of the corneal shape, results in irregular astigmatism with associated reduction in vision. It typically presents in adolescence and progresses in a variable manner. Keratoconus is a little-understood disease with an uncertain cause, and its progression following diagnosis is unpredictable. The mechanism of this extreme thinning appears to be related to increased proteinase activity, along with decreased proteinase inhibitors. Many biochemical and histopathological changes occur in the cornea in case of keratoconus which include epithelial changes as oedema of the basal cells especially at the base of the cone, swelling and fibrillar degeneration of Bowman’s layer, stromal thinning, rupture of Descemet’s membrane and flattening of endothelial cells. Several methods of measuring the corneal shape have been used. Corneal topography represents a significant advance in the measurement of corneal curvature over keratometry. Topography provides both a qualitative evaluation of corneal curvature. New technologies have met the demand for increased precision in evaluation of complex corneal shapes. These include Placido disk imaging, three dimensional topography, PAR (The posterior apical radius imaging device), slit-scanning topography, Scheimpflug imaging, ultrasound, and interferometric systems. The management of keratoconus varies depending on the state of progression of the disease. In very early cases, spectacles may provide adequate visual correction, but because spectacles do not conform to the unusual shape of the cornea and the resultant induced irregular astigmatism, contact lenses provide better correction. Contact lenses are the mainstay of therapy in this disorder and represent the treatment of choice in 90% of patients. The type of contact lens used varies depending on the stage of keratoconus. As the disease progresses rigid (hard) contact lenses become the mainstay of treatment. In the majority of eyes such lenses provide good visual rehabilitation. Unfortunately, they are not the solution in all cases. Discomfort and patient preference may limit the use of rigid contact lens wear and in advanced cases fitting may be problematic. Severe ectasia and central corneal scarring in advanced keratoconus can significantly limit the amount of visual rehabilitation achieved by rigid lenses. For these reasons, between 10-25% of patients with Keratoconus progress to a point where surgical intervention is required. Surgical options include: Intra-corneal ring segment insert (Intacs &Ferrara Rings), Riboflavin / Ultraviolet-A corneal cross linkage (CR3), Lenticular (lens) refractive surgery including refractive lens exchange with toric intraocular lenses & Corneal transplantation (or grafting) including penetrating keratoplasty and Lamellar keratoplasty. The development of intra-corneal ring segments has provided a surgical alternative to corneal transplantation in some eyes with keratoconus. This technology has been available for over 10 years and was initially developed for the correction of low degrees of myopia, up to -3.0 diopters, with some success. More recently, these ring segments have been used to reduce the irregularity of the cornea and flatten the apex of the cone in mild and moderate cases of keratoconus with some reported success. Intacs are inserted into the posterior stroma. The circular intra-lamellar pockets for the rings are created either using a specially designed vacuum lamellar dissector or with the femtosecond laser. There is some debate as to whether one or two intact segments should be inserted. Intacs are the treatment of choice in the contact lens intolerant eye, with keratometry less than 53 diopters and no central corneal scarring and should be attempted before considering DALK (Deep anterior lameller keratoplasty). It must be stressed, that intra-corneal ring technology does not offer a cure for the condition but can very often produce a marked improvement in unaided and best corrected visual acuity and allow eyes to be corrected with spectacles and/or soft rather than rigid lenses. The Ferrara Ring (FR) implant is mostly for keratoconic patients of any age with an evolving condition and intolerance to contact lenses or with sharp distortions in the cornea shape, which usually occur after transplants. The main purpose of FR is to reshape the cornea and thus reduce astigmatism, to reinforce and stabilize the cornea and possibly delay or prevent KC from progressing and to improve vision acuity. Corneal collagen cross-linkage (CR3) using Riboflavin/ ultraviolet–A light is a new therapeutic modality which may be the first available treatment to halt and stabilise the keratoconic process. Its aims are to increase the biomechanical stability of the corneal stroma, in terms of its tensile strength and its resistance to enzymatic digestion, by inducing and increasing cross-linkages between the stromal collagen fibres. The ability to halt disease progression at the earliest stages of the condition, when full visual rehabilitation can still be achieved with spectacles and soft contact lenses, offers great hope for future generations suffering with this not infrequent and often visually devastating condition. DALK negates the risk of endothelial rejection, improves postoperative biomechanical corneal stability& should reduce the risk of postoperative complications associated with intraocular surgeries. Many techniques are used to separate the stroma from Descemet’s membrane in lamellar keratoplasty including corneal injection with air, saline to turn the stroma opaque and so can be easily differentiated from Descemet’s membrane. Further refinements in operative techniques, together with improvements in technologies, such as the implementation of femtosecond lasers and mechanical microkeratomes for DALK, will allow refinement of lamellar techniques and improve the ease of performing these procedures for both surgeons and patients alike. Corneal transplant (penetrating keratoplasty) is the best and most successful surgical option for keratoconus patients who cannot tolerate contact lenses or are not adequately visually rehabilitated by them. Central scarring may preclude good vision from contact lenses, even when they are tolerated. A patient with keratoconus has an approximately 10–20% chance over his/her lifetime of needing a corneal transplant. The outcomes of PK for keratoconus are generally very good. Long-term studies have documented five and 10-year graft survival rates of over 90% in primary transplants. Increased rates of endothelial cell loss have been reported up to 17 years following PK, casting doubt on long-term graft survival. Conductive keratoplasty which entails applying radiofrequency energy to the corneal stroma may address significant astigmatism in eyes that cannot accept Intacs due to their corneal thickness or extreme K readings. LASIK has been used to treat myopic astigmatism in patients with keratoconus. The initial visual results appeared promising, but longer follow up revealed regression of the refractive outcome in some cases .Excessive thinning of the stromal bed together with the action of the intraocular pressure may cause a progressive keratectasia manifesting months after the LASIK. While many surgical interventions for keratoconus are still at an evolving stage, the use of techniques which are additive (such as Intacs), mechanically and chemically stabilize the cornea (riboflavin / UVA corneal collagen cross linkage), assumed to be neutral (conductive keratoplasty) or that replace the diseased corneal tissue (keratoplasty techniques) must be deemed preferable to procedures that remove tissue (excimer laser) and mechanically de-stabilize the cornea (astigmatic and radial keratotomies), particularly in a condition where the cornea is known to be thin, ectatic and mechanically unstable. Combination of different modalities may be done as CR3 with Intacs, LASIK after PKP or and toric phakic intraocular lenses for correction of residual myopia and astigmatism following Intacs or PKP. Thus it is important to emphasize that the development of new surgical treatment modalities and the refinement of older techniques with the introduction of new technologies offer the promise of improved visual outcomes for future generations with this condition. |